1. Field of the Invention
This invention relates to copolymers used as supports for emulsion polymerization. More particularly, this invention relates to alpha olefin maleic anhydride copolymers used as supports for emulsion polymerization.
2. Related Background Art
Aqueous emulsion polymerization is a process whereby water insoluble or mostly water insoluble monomers are polymerized in dispersed form in an aqueous medium. A surfactant or a surfactant blend is typically used to stabilize the prepared insoluble polymer particles, maintaining particle integrity and preventing phase separation.
Aqueous emulsion polymer systems have been developed in response to continuing environmental and regulatory pressures to reduce and/or eliminate the use of volatile organic solvents in polymer preparation and delivery systems. While emulsion polymerization has proven to be a highly successful and practical method of preparing commercial industrial polymer products, the performance characteristics of such systems, such as in coatings applications, often do not match the performance obtained from a polymer system delivered from an organic solvent. This is due in part to the existence of the surfactant in the emulsion polymer system. Comprised mostly of highly charged molecules, for example ionic type surfactants such as sodium lauryl sulfate, the surfactant contributes adversely to properties such as water resistance and detergent resistance in coatings applications.
Copolymerizable surfactants, fugitive surfactants, and reactive surfactants have been proposed in the patent and scientific literature as steps toward higher performing emulsion polymer systems. While these approaches have all met with some technical and commercial success, economic considerations often prevent these technologies from displacing conventional surfactants in emulsion polymerization.
An alternative technology is the use of polymeric materials as the stabilizing species in emulsion polymerization systems. These polymeric materials have been referred to in various publications as polymeric surfactants, protective colloids, polymer supports, support resins, polymer seeds, or polymeric stabilizers, and are typically lower molecular weight, between about 400-20,000 Daltons. The polymeric materials' functionality yields a polymer that is either water soluble or soluble on addition of base (known as alkali soluble resin, which contains carboxylic acid or sulfonic acid), or upon addition of acid (known as acid soluble resin, which contains nitrogen). These "polymeric stabilizers", as referred to herein, serve the same purpose as conventional surfactants in emulsion polymerization, and also provide additional performance characteristics not obtainable from conventional surfactants such as gloss development, flow and leveling control, and dry time control. Polymeric stabilizers are preferably prepared using weaker acids such as carboxylic acids instead of sulfonic acids, and utilize a fugitive amine, for example triethyl amine or ammonia, which results ultimately in a noncharged stabilizing component in the film. As explained below, the presence of large amounts of ionic species in final polymers can detract from performance characteristics such as water resistance. Systems containing polymeric stabilizers exhibit improved resistance properties including water resistance when compared to conventional stabilizers.
Examples of emulsion polymerization systems exist in the patent literature, whereby variations in the monomers, surfactants, addition profiles and processing conditions result in unique compositions and/or unique performance characteristics. U.S. Pat. Nos. 4,931,510, 5,013,794 and 5,258,466 disclose diisobutylene/maleic anhydride copolymers modified to imide with aminosulfonic acid which are used as resin supports in ranges of 2.5-30% by weight. The resins are used to polymerize such monomers as styrenics, (meth)acrylates, (meth)acrylonitriles and mixtures thereof. The resulting polymers are disclosed as useful as paper surface and fiber sizing agents.
U.S. Pat. No. 5,166,272 discloses an alpha olefin (C.sub.8, C.sub.10, C.sub.12) or undecylenic acid/maleic anhydride copolymer which has been modified to incorporate some olefin functionality used in an amount between 5% and 30% by weight to polymerize (meth)acrylics useful in coatings.
U.S. Pat. No. 4,775,723 discloses an alpha olefin (C.sub.6 -C.sub.40)/maleic anhydride copolymer, partially esterified, used in an amount of 2.5% to polymerize ethyl acrylate useful as a lubricant and emulsifying agent.
Japanese Patent No. 6,172,728 discloses an isobutylene/maleic anhydride copolymer modified to imide (commercial Japanese product Isobam 304.TM.) used in an amount of 13.8% by weight to polymerize vinyl acetate monomers which are used in adhesives. The patent discloses use of a co-stabilizer of modified polyvinyl alcohol. U.S. Pat. No. 5,298,568 discloses an alpha olefin/maleic anhydride copolymer modified with hydroxy and amino used in an amount of 81% by weight to polymerize acrylate monomers useful in inks and adhesives.
Japanese Patent No. 7,118,312 discloses a chlorinated alpha olefin/maleic anhydride copolymer used in an amount of 50% by weight to polymerize methacrylates useful as a pigment dispersion and in inks. Japanese Patent Nos. 51 001,706, and 83 013,679 disclose a nonpolymeric adduct of alpha olefin/maleic anhydride extended with diamine used in an amount of 20-80% by weight to polymerize (meth)acrylates and (meth)acrylonitriles which are useful as paper sizing agents. Although appearing similar, these adducts are actually non-polymeric emulsifiers and not properly characterized as polymeric stabilizers. Finally, Japanese Patent No. 6,088,052 discloses a diisobutylene/maleic anhydride copolymer, and partial esters thereof, used in an amount of 17% to 95% by weight to polymerize (meth)acrylates useful in ink, overprint and varnishes.
Although the published prior art teaches that polymers can be used as stabilizers in emulsion polymerization systems at levels as low as 5% by weight, experience has shown that these polymeric stabilizers are typically useful only at the level of 20 to 30 percent.
While the use of polymeric stabilizers generally enhances polymer performance, this relatively high level of stabilizer traditionally used often introduces some undesirable attributes for certain applications, including wet adhesion and long term water resistance.
Therefore an emulsion polymerization stabilizer is desirable which eliminates the need for volatile organic solvents in polymer preparation and delivery systems and is active at reduced levels from those of typical conventional and polymeric stabilizers.